1. Field of the Invention
The present invention relates to electrochemical methods and apparatus for producing ferrate (VI) compounds.
2. Description of the Related Art
Interest in the practical use of ferrate compounds has increased in the last two decades, mainly because of the strong oxidizing properties of ferrates. Additionally, the use of ferrate, FeO42−, promises a safe, convenient, and versatile alternative to current approaches for numerous industrial applications. One problem preventing the wide spread use for such processes is that ferrate is difficult to produce, particularly in commercial quantities, and current production methods produce a product typically containing residual impurities.
There are two basic methods for production of ferrate: chemical and electrochemical. Chemical methods contact an iron compound (i.e. iron (III) nitrate or iron (III) oxide) with an oxidizing material in either an alkaline environment (wet route) or under extreme temperatures in a controlled atmosphere (dry route). Electrochemical methods usually consist of a sacrificial iron anode placed in a proton exchange membrane (PEM) electrolyzer cell containing a strongly alkaline solution with an electrical current serving to oxidize the iron to Fe (VI).
Chemical production of ferrate typically uses a synthetic scheme involving a hypochlorite reaction. Most commonly, using alkaline oxidation of Fe (III), potassium ferrate (K2FeO4) is prepared via gaseous chlorine oxidation of ferric hydroxide in caustic soda, involving a hypochlorite intermediate. A number of difficulties are associated with the production of ferrate using this method. First, several requirements for reagent purity must be ensured for maximized ferrate yield and purity. However, even with these requirements satisfied, the purity of the potassium ferrate product still varies widely and depends upon many factors, such as reaction time, temperature, purity of reagents, and choice of isolation process. Ferrate prepared this way is typically 20% pure, with the major contaminants being alkali metal hydroxides, chlorides and ferric oxide. In addition, samples of this low purity product are unstable and readily decompose completely into ferric oxides.
Other chemical processes for preparation of ferrates are known and used, many of them also involving reactions with hypochlorite. Deininger discloses a chemical process for making stable, high-purity ferrate (VI) using beta-ferric oxide (beta-Fe2O3) and preferably monohydrated beta-ferric oxide (beta-Fe2O3—H2O), where the unused product stream can be recycled to the ferrate reactor for production of additional ferrate.
Mills, et al. disclose a method of making ferrate, involving a reaction with hypochlorite, as well as a method of stabilizing the ferrate product so that it can be used as an oxidizing agent.
Evrard, et al. disclose the preparation of alkali or alkaline earth metal ferrates that are stable and industrially usable as oxidizers, and the use of these ferrates for water treatment by oxidation. This method, however, introduces an additional impurity as sulfate compounds are utilized to stabilize the resulting ferrate.
The most overwhelming disadvantage to these processes is the use of hypochlorite. Although the ferrate ion, FeO42−, is an environmentally friendly oxidant itself, if the ferrate is produced by reaction with hypochlorite, its use will incur the deleterious side effects attributable to chlorine gas products.
Thompson discloses a method for direct preparation of iron and alkali metal or alkaline earth metal ferrates, where the iron in the product has a valence of +4 or +6. The method involves reacting iron oxide with an alkali metal oxide or peroxide in an oxygen free atmosphere or by reacting elemental iron with alkali metal peroxide in an oxygen free atmosphere. In addition, high temperatures are required (400°–700° C.) and an impure product is obtained.
Electrochemical oxidation of iron to ferrate (VI) has been given more attention in recent years. This method has the advantage of not using chemical oxidizers, such as hypochlorite, that add impurities to the ferrate product and have a negative environmental impact. FIG. 1 is an exploded view of a prior art electrochemical method using an ion-transfer membrane (such as perfluoronated sulfonic acid polymer membrane) that separates the anode and cathode chambers. The anode is sacrificial, usually consisting of tightly wound iron wire (either pure iron or carbon steel) and the cathode can be constructed of one of several materials, including porous carbon, nickel, or even carbon steel. A concentrated sodium hydroxide solution is pumped from a reservoir into the base of the anode chamber and collected from the top of the chamber. In a similar manner, a sodium hydroxide solution is passed through the cathode chamber. An electrical current is applied across the cell, causing the iron anode to oxidize to Fe(VI), which is soluble in sodium hydroxide and is carried off in the flowing anolyte.
In a series of patents, Deininger, et al. disclose an electrochemical method for ferrate production using a dual chamber cell, similar to that shown in FIG. 1, that is separated by a cation exchange membrane and a concentrated sodium hydroxide solution used for the anode and cathode solutions, with the anolyte also containing a sodium halide. The source of ferric ions can come from a ferric salt, iron scrap, or an iron anode. An electrical current is applied to the cell and the anolyte and catholyte solutions are flowed through the chambers. Optionally, the electrochemical cell may be operated with no flow of the hydroxide solutions.
Bouzek, et al. have also studied the electrochemical production of ferrate using an apparatus similar to Deininger's. In their process, Bouzek, et al. use a dual chamber cell with the anode and cathode chambers separated by a porous membrane and the cell is designed to operate with no flow of the electrolyte solutions. Various iron compounds and alloys were studied as well as the current density and temperature of reaction in order to determine the optimal conditions for ferrate production.
A primary disadvantage of these methods is that they also require several additional steps in order to obtain a solid ferrate salt. It is difficult to obtain solid ferrate salts because ferrate salts are soluble at greater concentrations in sodium hydroxide, even at low temperatures, than what is typically produced by either chemical or electrochemical processes. Most often, the solution of sodium ferrate is produced in sodium hydroxide and then, in a separate step, saturated with potassium hydroxide, resulting in a slurry of relatively insoluble potassium ferrate in a strongly alkaline solution. This slurry can be separated to obtain a raw ferrate/hydroxide sludge that can then be purified by one of several conventional methods. The remaining hydroxide contains too much KOH to be recycled for further ferrate production because the presence of KOH would cause precipitate fouling of the membrane and clogging of the anode chamber. As a consequence, the remaining mixed hydroxide solution must be discarded at very high disposal cost.
Finally, while these processes are operable for very small-scale production of ferrate, they present multiple difficulties for large-scale generation of Fe(VI) compounds. For example, during ferrate production some Fe(VI) degrades to Fe(III), which is insoluble in hydroxide solutions. The Fe(III) precipitates out of solution and coats the walls of the anode chamber as well as the separating membrane. As the membrane is coated, the current efficiency and production rate decrease until ferrate generation is less than ferrate decomposition. In order to prevent this, the production must be frequently stopped, the cell drained and cleaned with acid, and the cell refilled with either a fresh NaOH solution or by the previous ferrate/NaOH solution before production can be resumed. Additionally, the most efficient processes use expensive ion exchange membranes, which are unfeasible for industrial-scale processes.
Consequently, commercial supplies of ferrate are almost nonexistent. Despite the tremendous potential for ferrate in many industrial processes, the current production methods are insufficient and prohibitively expensive, making large-scale use of ferrate impractical.
Therefore there is a need for an improved method of producing ferrate (VI). It would be desirable if the method produced ferrate in a continuous process that lends itself to production of commercial quantities of ferrate. It would also be desirable if the method used inexpensive materials and made efficient use of solutions to minimize waste products.